Remember how one of the main benefits of quest heavy systems was providing interesting stories to make people feel like their actions had meaning? Yeah... about those... after you clear out your first quest hub or two, they start to blend together. You start skimming the text to see what kind of rat tails you need to gather (whether its orc hearts or dragon scales, in the end they are all rat tails).
No MMO can realistically create thousands of interesting quest lines and stories. They simply do not have the staff to put into that enormous amount of high quality, creative content. Most MMOs have a handful of truly interesting and engaging quest lines, and they are certainly "don't miss" aspects of game content. But the overwhelming majority of quests are repetitive and uninteresting from a story perspective. And once you start ignoring the quest text to just skip to the objectives, you run the risk of missing out on the ones that are actually good. You develop a habit of skipping the story, so the 1 out of 100 that you would have liked get lost in the shuffle.
Wolfshead, author of Wolfshead Online and a regular readers of Muckbeast, summed this concept up succinctly:
"They will never experience the autonomy of being able to explore a world without a questgiver telling them what they must do."
In quest heavy worlds, the game teaches you that you shouldn't go somewhere unless you have already loaded up on the quests for it. Also, since quest NPCs are not always at the entrance of a zone, you generally have to do research on fan sites to make sure you have all the quests before you even go to a new location. Otherwise, you will find yourself backtracking to visit locations you already fully explored simply to complete or continue a quest chain.
Eventually, quests teach players not to explore for the sheer joy of it, but to follow the path mapped out by the developers through their quest chains. Some WoW Add-Ons even put a giant yellow arrow in the middle of the screen that constantly points you to your next objective. People end up just watching and following the arrow and barely even see the scenery.
Exploring for its own sake was one of the original joys of MMOs. Exploring for a quest or a reward would be far more enjoyable if it was secondary to the exploration itself.